1. Field of the Invention
The present invention relates to an image processing apparatus, a printing apparatus and an image processing method, and in particular to color reproduction of a primary color such as cyan, magenta or yellow.
2. Description of the Related Art
An image forming apparatus that forms image information (including characters, symbols and the like) on a print medium is known as a printer, a copier, and a facsimile machine or an output device of information processed by a computer or a word processor. Such an image processing apparatus performs the printing based upon image information with use of a printing mechanism, on the print medium such as papers or plastic thin sheets (OHP sheet). Examples representative of these printing systems include an ink jet system, a wire dot system, a thermo-sensitive system, a thermal-transfer system, an electro-photographic system using a laser beam and the like, and the like.
Among these printing systems described above, the ink jet system ejects ink from a print head in the form of drop to be landed to the print medium for forming dots to generate an image thereon. An ink jet printer using this system is widely used recently as an output device for an information processing equipment such as a computer or the like.
In the ink jet printer, dyes or pigments are used as coloring materials contained in the ink as recording materials. Basic colors of cyan (C), magenta (M) and yellow (Y) as three primary colors of the subtractive color mixing are used as ink colors by the coloring materials. Also, a system using four colors containing further black (K) is generally used. This system can realize reduction in an ink applying amount to the print medium or the density improvement, by further using black. Further, in a system for a high image quality of a photographic image, the ink of light cyan (LC) and light magenta (LM), which have lower concentration than that of cyan and magenta inks, may be used in addition to the ink of the above four colors. This allows the image quality improvement such as reducing a granular feeling on a highlight part of the printed image. Also, print systems further using ink of gray (Gry) and light gray (light Gry) are known.
For obtaining an image with a desired image quality by using such various kinds of ink, color separation processing for defining what combination of ink is required for realizing the color represented by the image information becomes important.
Japanese Patent Laid-Open No. 2003-039714 and Japanese Patent Laid-Open No. 10-112809 (1998) discloses color separation processing in a case of using dark and light inks. Japanese Patent Laid-Open No. 2003-039714 discloses color separation which achieves color reproduction of a primary color such as cyan or magenta with both of the dark and light inks for the aim of decreasing deterioration of gradation sequence properties due to a change of an ink ejection amount at the time of switching ink between dark and light inks. In addition, Japanese Patent Laid-Open No. 10-112809 (1998) discloses color separation processing for reproducing a primary color with both of dark and light inks, for decreasing uneven gloss generated due to presence or absence of bronze in a case of using both light ink which easily causes a bronze phenomena and dark ink which hardly causes a bronze phenomena.
In the meantime, a demand for higher image quality is recently increasing along with wide use of a color printer. However, basic color recording materials of C, M and Y or quasi-basic color recording materials such as LC, LM, Gry, and K have limitations in reproducing colors, due to coloring characteristics of the recording materials and therefore it is hard to reproduce a brighter color. Consequently, the recording materials of particular colors such as red (R), orange (O), green (G), blue (B) and violet (V) in addition to the basic colors and the quasi-basic colors become to be more used.
Color separation processing for the case of using such particular colors is disclosed in the following documents.
Japanese Patent Laid-Open No. 10-112809 (1998) discloses that with regard to basic color recording materials produced by color separation, color separation in a system of using particular colors is made by exchanging two of the basic colors for the particular color obtained by mixing the two basic color recording materials.
Further, Japanese Patent Laid-Open No. 2004-314490, Japanese Patent No. 02711081, Japanese Patent Laid-Open No. 2005-335191, Japanese Patent Laid-Open No. 2005-059362 and Japanese Patent Laid-Open No. 2005-217978 disclose color separation (separation processing) which aims at reduction of a granular feeling generated in frequent use of particular colors in the case that basic colors of C, M, and Y, K, and quasi-basic colors (chromatic primary color) such as LC, LM, and particular colors (chromatic secondary color) are used. In particular, these documents disclose the color separation in which color reproduction property is effectively used while a particular color ink use amount in a light color reproduction region is suppressed.
Further more, Japanese Patent Laid-Open No. 2005-217987 and Japanese Patent Laid-Open No. 2002-033930 disclose the color separation in which, in the case of using particular color ink having the coloring property of high lightness and high-chroma, the particular color ink is used only in a high lightness region at which expanding of a color gamut is effectively made by the particular color ink, and reversely the particular color ink is not used in a low lightness region at which the particular ink causes a reduction in a color gamut.
In the mean time, as the characteristics of ink which is a recording material of the ink jet printer, various characteristics such as coloring characteristics and besides, an effect of bleeding to characters and ruled lines, long-term preservation stability, and ejection stability are considered. Here, the long-term preservation stability includes characteristics such as a resistance to exposure to ozone, a resistance to exposure to ultraviolet, and water resistant, and the ejection stability means that an ejection failure such as clogging do not occur. Such ink is required to be manufactured in consideration of not only the coloring characteristic but also various characteristics except for it. Therefore, there is frequently used the ink for which the requirement on the coloring characteristic is compromised to some degrees.
FIG. 1 is a diagram showing color reproduction properties of the inks for which the requirement on the coloring characteristics is compromised to some degrees. FIG. 1 shows a change in reproduction color in accordance with an ink applying amount of each of basic color ink of C, M and Y, quasi-basic color ink of LC and LM and particular color ink of R and G, which are orthogonally projected on an a* b* plane of a CIE-L* a* b* color space. As shown in FIG. 1, a chroma of each ink color increases from a paper white point W corresponding to the ground color of the paper in the vicinity of an origin of the a* b* plane, as an applying amount of each ink increases. The quasi-basic color inks LC and LM respectively reproduces nearly the same hue to that of the corresponding basic color inks C and M, but each reproduces lower chroma than that of the corresponding basic color ink C and M. The particular color ink G reproduces a color between color reproduction lines of Y ink and C (LC) ink, and the particular color R reproduces a color between color reproduction lines of Y ink and M (LM) ink. Further, Y ink realizes color reproduction in which the hue is maintained along the positive direction of a b* axis, while the chroma is increasing.
However, in an example shown in FIG. 1, color reproduction line 1201 by C ink and color reproduction line 1203 by M ink each have portions 1201A and 1203A where the hue largely bends (shifts) from cyan to blue in the portion 1201A and magenta to red in the portion 1203A, with an increase of the ink applying amount. Also, light color inks LC and LM in the same kind of colors as the C and M respectively have portions where the hue shifts likewise. However, the hue shift amount is relatively small in the case of the light color ink.
Hereinafter, the shift of the hue will be explained with reference to ink C as an example.
FIG. 2 is a diagram showing a color reproduction region of a device (for example, display) using color signals R, G and B as image information inputted to a printer. The device color shown in a combination of 8 bits of each of the color signals R, G and B is represented as one point in the cube shown in FIG. 2.
Consideration is made to a case where, with regard to color separation of a device color on the White-Cyan line 41 of the cube shown in FIG. 2, a single color of the ink C monotonically increases from W (white) point to C (Cyan) point. In this case, the color separation of the device color on B (blue)-C (Cyan)-G (Green) line shown in FIG. 2 is that, as shown in FIG. 4, an ink amount of C ink is constant from B point via C point to G point with respect to C ink. In addition, from B point to C point, M ink monotonically decreases and becomes 0 at C point, and from G point to C point, Y ink monotonically decreases and becomes 0 at C point.
FIG. 5 is a diagram showing a color reproduction region of a printer in a case of executing simple color separation processing shown in FIG. 3. FIG. 5 is a diagram similar to FIG. 1 and represents the color reproduction region in an orthogonal projection to the a* b* plane. FIG. 5 has L* axis showing lightness which is in a direction vertical to the plane of the drawing sheet, a* axis which is in a lateral direction in FIG. 5 and b* axis which is in a longitudinal direction. In FIG. 5, a solid line shows, among the color reproduction region of the printer, a color reproduction line in the lighter (W) side, and a broken line shows a color reproduction line in the darker (K) side. More specifically, in the cube shown in FIG. 2, the color reproduction line of each line for connecting W with each vertex C, G, Y, R, M and B is in the lighter side and the color reproduction line of each line for connecting K with each vertex C, G, Y, R, M and B is in the darker side.
As shown in FIG. 5, the color reproduction line 1201 from W to C has the characteristics in which the hue, as explained with reference to FIG. 1, largely shifts as C increases and the chroma reaches the maximum and thereafter decreases. On the other hand, the color reproduction line 1202 from C to K (black) has no large shift of the hue. This is because, on the color reproduction line 1202 between C and K, the lightness and the hue are adjusted at the same time by balance of complementary color components, such as M and Y, to C, and the hue shift on the line is restricted as compared to on the color reproduction line 1201 between W and C which is formed of the single color of C ink.
FIG. 6 shows the color reproduction region of a W-C-B plane among the color reproduction region shown in FIG. 5, which is surrounded by a bold solid line. In addition, FIG. 7 shows the color reproduction region of a C-B-K plane among the color reproduction region shown in FIG. 5, which is surrounded by a bold solid line. It is seen in these figures that an area of the upper side surface of the C-B line is largely different from an area of the lower side surface of the C-B line, and the W-C-K surface is largely distorted.
More specifically, the inventor of the present invention has found out that in a case of using basic color recording material for which the requirement on the coloring characteristics is compromised to some degrees and performing simple color separating, as described above, the following problem occurs. When a gradation image along the W-C-K line or a gradation image in contact with the W-C-K surface is printed, the region reproduced in a cyan color is mixed with the region reproduced in a blue color to generate a false gradation sequence in a color tone. Then, the inventor has found out that, for avoiding such false gradation sequence, it is required to compensate for color reproduction characteristics of the basic color recording material, on color reproduction of a device primary color line (for example, W-C line in FIG. 2 as described above) using a basic color recording material (for example, W-C line in FIG. 2 as described above).
In color separation described in each patent document described above, the above stated compensation is not sufficient or is not disclosed.
More specially, in a method of reproducing a device primary color using both of C ink and LC ink similar in a kind of color to C ink, which is described in Japanese Patent Laid-Open No. 2003-039714, color separation of the color on the W-C line 41 shown in FIG. 2 is such as is shown in FIG. 8. As shown in FIG. 8, LC ink increases from W point toward C point, and C ink monotonically increases as well as LC ink decreases nearly from the halfway between W point and C point where the granular feeling becomes not to be noticeable to reproduce the color of C point by using both of C ink and LC ink. In addition, with regard to the color separation of colors on B-C-G line shown in FIG. 2, C ink is, as shown in FIG. 9, constant from B point via C point to G point. In addition, LC ink increases and decreases like a mountain in the vicinity of C point. Further, M ink monotonically decreases from B point to C point and becomes 0 at C point, and Y ink monotonically decreases from G point to C point and becomes 0 at C point.
However, the color separation described in Japanese Patent Laid-Open No. 2003-039714 can make a slight color adjustment only, and can not compensate for a large hue shift in C line 1201 (refer to FIGS. 5 to 7). More specifically, in the case of manufacturing recording materials of the nearly same color such as C and LC, similar coloring materials are used in many cases. Therefore, in a case where the color reproduction in C ink generates the hue shift largely, the color reproduction in LC ink, although the shift degree is different, similarly generates the hue shift. As a result, the compensation effect for the hue shift is lowered.
The color separation using a basic color and a particular color, described in Japanese Patent Laid-Open No. 10-112809 (1998) does not consider compensation for the coloring characteristic of a coloring material itself of the basic color. In this case, the color separation of the device colors on W-C line 41 shown in FIG. 2 may have a monotonic increase of a single color of C ink from W point to C point. In this condition, when using the recording material for which the requirement on coloring characteristics is compromised to some degrees, a large hue shift on line 1201 shown in FIGS. 5 to 7 may occur.
Japanese Patent Laid-Open Nos. 2004-314490, 2005-335191, 2005-059362 and 2005-217987 disclose the color separation in which the particular color is not used in a highlight portion for reduction of a granular feeling in color reproduction of a device secondary color. These patent documents disclose the color separation of the device secondary color where the particular color is frequently used, but does not disclose the color separation of the device primary color at all.
Likewise, Japanese Patent Laid-Open Nos. 2005-217987 or 2005-059361 disclose the color separation in a case of using particular color ink of the coloring characteristics in which the lightness is high and the chroma is high in a device secondary color, but does not disclose the color separation of the device primary color at all. That is, these patent documents do not suggest teachings of compensation for the hue shift in the color separation of the device primary color.
On the other hand, a color separation in which a use amount of basic color ink is controlled to the extent that the hue shift does not occur can be considered as follows, as the color separation which is not disclosed in the patent documents described above.
In this color separation, though the color separation of W-C line 41 shown in FIG. 2 shows a monotonic increase of a single color of C ink from W point to C point, similarly to such as shown in FIG. 3, a C ink amount at C point becomes smaller than a printable ejection amount. In this case, the color separation of B-C-G line shown in FIG. 2, which forms the outermost portion of the color reproduction region, is that with respect to M and Y inks, similarly to that in FIG. 4, M ink monotonically decreases from B point to C point to become 0 at C point, and Y ink monotonically decreases from G point to C point to become 0 at C point. On the other hand, C ink has the printable limit amount by the single color of C ink at respective B and G points, and becomes smaller than the printable ink ejection amount at C point. In consequence, an ink use amount of C ink is shown in a constant line between B, C and G in FIG. 4, and on the other hand, in the present example, in a downwardly convex arc line from B point via C point which is a trough to G point. Such color separation realizes a color reproduction similar to that of the color reproduction line 1201 formed of C ink only on the a* b* plane shown in FIG. 10 or on the C* L* plane shown in FIG. 11, but the terminal of this line becomes point 1201B to the halfway of line 1201. In this case, as seen in FIG. 10, though an increase of the hue shift can be avoided, as shown in FIG. 11, a value of a reproducible chroma C* is reduced. That is, simple control of the use amount of the basic color ink thus does not prevent reduction in chroma of the device primary color, raising the problem that the color reproduction region becomes small.
Likewise, as color separation which is not disclosed in each patent document described above, it is considered that a basic color ink causing the hue shift in a high chroma portion is adjusted in color using the other basic color ink. In such a color separation of W-C line 41 shown in FIG. 2, as shown in FIG. 12, C ink monotonically increases from W point to C point, and Y ink starts to be used for color adjustment from the halfway of W-C line and monotonically increases. Then, C ink and Y ink are mixed at C point. It should be noted that, in the color separation of the B-C-G line in FIG. 2 of this case, as shown in FIG. 13, C ink is constant form B point via C point to G point. In addition, M ink monotonically decreases from B point to C point to become 0 at C point and Y ink monotonically decreases from G point to the halfway between C point and B point, and at C point C ink and Y ink are, as described above, mixed and becomes 0 in the halfway between C point and B point.
The color reproduction region of W-C line 41 by this color separation is shown in reproduction line 1602 (the end of the line is shown by a reference sign 1602A in FIG. 10) in FIG. 10 or 11. As seen in FIG. 10, the hue shift can be controlled, but as shown in FIG. 11, the chroma is largely reduced as compared to reproduction line 1201 in the case of using the single color of C ink. That is, in a case of reproducing the device primary color by mixing a plurality of basic color ink, there occurs the problem that the chroma of the device primary color is reduced to make the color reproduction region small.